1. Field of the Invention
The present invention relates to a fixing apparatus and an image forming apparatus.
More specifically, the present invention relates to a fixing apparatus comprising, at least, a heating element, an electric power supplying portion (heating means) for supplying an electric power to the heating element, at least one or more temperature detecting means, a first rotary member shifted together with a recording material and a second rotary member for cooperating with the recording material to define an abutment portion therebetween and wherein a temperature of the first rotary member is controlled by performing feedback control of the electric power supplied from the electric power supplying portion to the heating element on the basis of a temperature detected by the temperature detecting means and the recording material bearing an image is nipped and conveyed at the abutment portion thereby to heat the recording material, and an image forming apparatus, for example, such as an electro-photographic copier, printer, facsimile and the like, having such a fixing apparatus.
2. Related Background Art
In recent years, colorization in an image forming apparatus such as a printer, a copier and the like has been progressed. As a fixing apparatus used in such an image forming apparatus, a fixing apparatus of heat roller type including a fixing member having an elastic layer is well known.
However, in the fixing apparatus of heat roller type having the elastic layer, there was a problem that a heat capacity of the heat roller itself becomes great and a time (warm-up time) required for increasing a temperature of a fixing roller up to a temperature suitable for toner image fixing becomes long, and, further, the cost of the fixing apparatus was increased.
As a fixing apparatus having short warm-up time, a fixing apparatus of belt fixing time widely used in a monochromatic image forming apparatus is well known. A schematic constructional view showing one example of such a fixing apparatus is illustrated in FIG. 24.
The fixing apparatus is generally designated by the reference numeral 201. A fixing belt unit 202 is an assembly comprising a heat folder 207 having a half circular bucket-shaped cross-section, a fixing heater 204 secured to a lower surface of the heater holder 207 along a longitudinal direction of the heater holder (perpendicular to the plane of FIG. 24) and a fixing belt 203 having an endless thin layer and loosely mounted around the heater holder 207 having the fixing heater.
An elastic pressurizing roller 205 has metal core both ends of which are rotatably supported between side plates of the fixing apparatus.
The fixing belt unit 202 is disposed on the elastic pressurizing roller 205 in parallel with the pressurizing roller 205 with the fixing heater 204 facing downwardly, and both ends of the heater holder, 207 are pushed upwardly with a predetermined urging force by means of biasing means (not shown). With this arrangement, a fixing nip portion 206 having a predetermined width is formed by abutting a lower surface of the fixing heater 204 against an upper surface of the elastic pressurizing roller 205 with the interposition of the fixing belt 203 in opposition to elasticity of the pressurizing roller.
The elastic pressurizing roller 205 is rotatingly driven at a predetermined peripheral speed by means of a driving mechanism (not shown) in a direction shown by the arrow. By such rotation of the elastic pressurizing roller 205, a rotational force is applied to the fixing belt 203 at the fixing nip portion 206 by a friction force between the elastic pressurizing roller 205 and an outer peripheral surface of the fixing belt 203, with the fixing belt 203 is rotatingly driven around the heater holder 207 at a peripheral speed substantially corresponding to the peripheral speed of the elastic pressurizing roller 205 in a direction shown by the arrow in a condition that an inner surface of the fixing belt is closely contacted with and slid on the lower surface of the fixing heater 204 at the fixing nip portion 206.
The fixing belt 203 is constituted by a heat-resistive resin endless belt having a thickness of about 50 μm and a mold releasing layer (coating layer made of fluoro-resin or the like) formed on the endless belt and having a thickness of 10 μm. Further, in order to reduce a heat capacity of the fixing belt 203, any elastic layer is not provided on the fixing belt 203.
The fixing heater 204 is constituted by a resistive heating element formed on a ceramic substrate. Temperature detecting means 209 is contacted with a rear surface of the fixing heater 204 to detect a temperature of the fixing heater 204, and temperature control is performed by means of control means (not shown) by controlling an electric power supplied to the fixing heater 204 so that the temperature of the fixing heater 204 becomes a desired temperature.
In a condition that the elastic pressurizing roller 205 is rotatingly driven and the fixing belt 203 is rotatingly driven and the fixing heater 204 is started-up to the predetermined temperature to be subjected to the temperature control, a recording material P bearing a unfixed toner image t is introduced between the fixing belt 203 and the elastic pressurizing roller 205 at the fixing nip portion 206. The recording material P together with the fixing belt 203 is pinched and conveyed at the fixing nip portion 206 while closely contacting a surface of the recording material bearing the unfixed toner image with the outer surface of the fixing belt 203. During such pinching and conveyance, heat from the fixing heater 204 is applied to the recording material P via the fixing belt 203 and the recording material is subjected to pressure from the fixing nip portion 206, with the result that the unfixed toner image t is fixed onto the recording material P by heat and pressure as a permanent fixed image. After passed through the fixing nip portion 206, the recording material P is self-stripped from the surface of the fixing belt 203 by curvature and then is discharged.
In the fixing apparatus 201 having the above-mentioned construction, since the heat capacity of the fixing belt 203 is very small, after the electric power is applied to the fixing heater 204, the temperature of the fixing nip portion 206 can be increased, for a short time, up to a temperature capable of fixing the toner image.
However, in a case where the belt type fixing apparatus 201 using the fixing belt 203 having no elastic layer is used as a fixing apparatus of a color image forming apparatus, since the fixing belt 203 as the fixing member has no elastic layer, the surface of the fixing belt 203 cannot follow unevenness of the surface of the recording material, unevenness caused by presence/absence of the toner layer and unevenness of the toner layer itself, thereby causing difference in heat amount supplied from the fixing belt 203 between the recessed portions and the protruded portions. In the protruded portions adequately contacted with the fixing belt 203, since the heat from the fixing belt 203 is well transferred, the great heat amount is given; while, in the recessed portions which are not contacted with the fixing belt 203 so well, since the heat from the fixing belt 203 is hard to be transferred in comparison with the protruded portions, given heat amount is small. As such, since the heat amount is differentiated due to the unevenness of the toner layer, a fusing condition of the toner becomes uneven, thereby generating gloss irregularity, which affects an influence upon the fixed image.
Particularly, in the color image, since plural color toner images are overlapped and color-mixed, the unevenness of the toner layer is greater in comparison with the monochromatic image, and, thus, in the case where the fixing belt 203 has no elastic layer, gloss irregularity of the fixed image becomes greater, thereby worsening image quality. Further, in a case where the recording material P is an OHP sheet, when the fixed image is projected, light scattering is generated due to the fact that the surface of the fixed image is not microscopically uniform, thereby resulting in reduction in permeability.
Although it can be considered that the fixing belt 203 having no elastic layer is used and silicone oil is coated on the fixing belt 203 so that the heat is well transferred onto the unevenness of the recording material P and the unfixed toner image t, there were problems that the cost is increased and that the fixed image and the recording material P are contaminated by oil.
To avoid this, a fixing apparatus constituting a low cost color on demand fixing apparatus by using a fixing belt having an elastic layer in a belt fixing apparatus has been proposed (for example, refer to Japanese Patent Application Laid-Open No. 11-15303).
FIG. 25 is a schematic constructional view of a belt fixing apparatus using a fixing belt 203 having an elastic layer as a fixing member. Constructional members and parts common to those in FIG. 24 are designated by the same reference numerals and explanation thereof will be omitted.
In a case where this fixing apparatus is used, since heat transfer characteristics of silicone rubber used in the elastic layer of the fixing belt 203 is small, temperature response of the fixing belt 203 is bad and an increasing speed of a temperature of a fixing sleeve is very slow in comparison with the temperature increasing speed of the fixing heater 204. Further, a difference between the temperature of the fixing heater 204 and the temperature of the fixing belt 203 reaches several tens of ° C. at the maximum, and, such temperature difference is greatly differentiated between idle-rotation and sheet passing. Thus, as is in conventional cases, in a system in which the temperature detecting element 209 is disposed on the rear surface of the fixing heater 204, it was very difficult to control the temperature of the fixing belt 203.
To avoid this, in the apparatus shown in FIG. 25, the temperature detecting element 209 is disposed on the inner surface or front surface of the fixing belt 203 rather than the fixing heater 204 to detect the temperature of the fixing belt 203 itself and the temperature control of the fixing belt 203 is performed by controlling the temperature of the fixing heater 204 by feedback control such as PID control.
By using such a construction, the temperature of the fixing belt 203 can be controlled more accurately.
However, as a result that a fixing operation was performed by using this fixing apparatus, the following two problems occurred.
(1) Problem 1
From a condition that constant temperature control is performed to stabilize the temperature, when the recording material P is passed, immediately after the recording material P rushes into the fixing nip portion 206, the temperature of the fixing belt 203 is decreased abruptly, and, thereafter, although the temperature is increased, overshoot is generated, thereby resulting in great temperature fluctuation.
This phenomenon will now be explained.
In the conventional fixing apparatus, after PID control is continued in an idle rotation condition, the recording material P is passed. In this case, as shown in FIG. 5, immediately after the recording material P conveys into the fixing nip portion 206, the temperature of the inner surface of the fixing belt 203 is decreased abruptly, and, thereafter, although the temperature is increased, overshoot is generated. Concretely, as shown in FIG. 5, in the idle rotation before the sheet passing, although the temperature of the inner surface of the fixing sleeve 203 is stabilized, immediately after the recording material P conveys into the fixing nip portion 206, it takes about 10 seconds until the temperature of the inner surface of the fixing belt 203 is decreased up to a temperature lower than a target temperature by about 10° C. and then is increased up to a temperature greater than the target temperature by about 7° C. and then is stabilized. During a period from immediately after the recording material P conveys into the fixing nip portion 206 to when the temperature of the inner surface of the fixing sleeve 203 is stabilized, temperature ripple (difference between a maximum temperature value and a minimum temperature value within a given time) was about 17° C. In subsequent continuous sheet passing, stable temperature control (temperature ripple of about 60° C.) was achieved.
In the condition that the temperature of the inner surface of the fixing sleeve 203 is stabilized during the idle rotation, the electric power being applied to the fixing heater 204 is about 80 W, and in the condition that the temperature of the inner surface of the fixing sleeve 203 is stabilized in the subsequent continuous sheet passing, the electric power being applied to the fixing heater 204 is about 300 W which is substantially constant.
On the other hand, immediately after the recording material P conveys into the fixing nip portion 206, behavior that the electric power is increased up to about 500 W due to the temperature reduction and fluctuates by about 20 seconds and then is stabilized at about 300 W was indicated. This phenomenon could not be solved even when each gain in the PID control is adjusted, and it was difficult suppress the temperature fluctuation for more.
In this way, when the temperature fluctuation of about 17° C. is generated, in a color image forming apparatus of electro-photographic type used in the test, the fixing abilities of the outputted prints were greatly dispersed and gloss properties were greatly changed thereby to worsen image quality. Furthermore, depending upon the recording material and/or printing pattern, poor fixing occurred due to great temperature fluctuation.
Further, this phenomenon became more and more conspicuous as a process speed was increased, and, in the measurement of FIG. , although the process speed was 87 mm/sec, when the process speed was 190 mm/sec, the temperature of the inner surface of the fixing belt 203 was decreased to a value lower than the target temperature by about 20° C. immediately after the recording material P conveys into the fixing nip portion 206, and thereafter, the temperature was increased to a value greater than the target temperature by about 8° C. to provide temperature nipple of about 28° C. In this case, the fluctuation range of the gloss property is further increased and partial poor fixing is also worsened.
(2) Problem 2
In the start-up, the overshoot is great. Further, in the start-up, the temperature nipple is great, and, after the start-up and upon the recording material rushing, dispersion of temperature becomes great.
This phenomenon will now be explained.
In this fixing apparatus, start-up temperature control sequence is constituted by the following two steps a and b.    a. “start-up ((fix) electric power control”; and    b. “PID control”
The “start-up electric power control” in the step a is control in which a constant electric power is applied in order to quickly start-up the temperature of the fixing apparatus and to ensure on-demand, and, in this example, the electric power of 1000 W is applied to the fixing heater 204. In this case, the fixing belt 203 is heated by the fixing heater 204 while being driven by the rotation of the pressurizing roller 205. When the detection temperature of the temperature detecting means 209 reaches a predetermined temperature (target temperature of −20° C.: thus, for example, when the target temperature is 190° C., 190° C.−20° C.=170° C.), the sequence goes to the “PID control” in the step b, where the electric power applied to the fixing heater 204 is controlled the temperature of the rear surface of the fixing belt 203 approaches the target temperature by means of the PID control.
When the above controls are used, as shown in FIG. 21, the overshoot is generated and the temperature ripple due to the overshoot becomes great.
If the overshoot and the temperature ripple become great in the start-up, the following two problems will occur.
I. If operations at the high temperature due to the overshoot in the start-up are repeated, damage on various parts of the fixing apparatus becomes great, thereby shortening a service life of the fixing apparatus.
II. Since the temperature nipple is great in the start-up, at the moment when the recording material P enters into the nip portion, the temperature is not stabilized, and, since the temperature fluctuation becomes great while the recording material P is passing through the fixing nip portion 206, gloss fluctuation in each outputted print becomes great, which results in an unfavorable image. Further, in dependence upon the kind of the recording material P and/or printing pattern, poor fixing will occur at areas where the temperature is decreased.
On the other hand, it is possible to suppress the overshoot more or less by starting-up the temperature gently with a small electric power. In this case, however, it takes a long term until the fixing apparatus is started-up to the predetermined temperature, thereby worsening the on-demand property.
In this way, when the conventional fixing apparatus controlling method is used, there is a trade-off relationship between the on-demand property and the stability of the temperature control.
The Inventors zealously investigated such a phenomenon and, it was found that these problems are generated by the following two reasons:
1) Since heat transfer characteristics of the silicone rubber used in the elastic layer of the fixing belt 203 is small and the heat capacity is great, response is worsened during a period from when the electric power is applied to the fixing heater 204 and to when the temperature of the fixing belt 203 is increased.
2) Since the position of the temperature detecting element 209 for detecting the temperature of the rear surface of the fixing belt 203 is spaced apart from the fixing heater 204 as the heating member, detection timing is delayed.
That is to say, in the feedback control as represented by the PID control, since change in the control amount is detected and an operating amount corresponding thereto is added, if the temperature control tries to be performed on the basis of the PID control, time lag due to the above two reasons 1) and 2) becomes great, and, if the change in temperature is great as is in the start-up and the recording material P rushing, even when the detection result of the temperature detecting element 209 is reflected upon the electric power control, since the actual temperatures of the fixing heater 204 and the fixing nip portion 206 have already different values, correct temperature control cannot be achieved, with the result that the overshoot and hunting (temperature ripple) are apt to occur.